This invention relates generally to superconducting materials, and in particular, to a process for making electrical connections to and between such materials. Superconductors are materials having essentially zero resistance to the flow of electrons at temperatures below a certain critical temperature, T.sub.c. The T.sub.c varies for each type of superconducting material.
It is known that certain metal oxide ceramics exhibit superconductivity at a relatively high temperature, i.e., at temperatures greater than 30.degree. K. (0.degree. K=-273.15.degree. C.). The recently discovered superconductors can be categorized into two groups: the first having the general formulae of R.sub.a M.sub.b X.sub.c O.sub.y ; and the second having the general formula (M.sub.1-x Q.sub.x).sub.a L.sub.b Ca.sub.c Cu.sub.d O.sub.y. In the superconductors having the formula R.sub.a M.sub.b X.sub.c O.sub.y, the symbols R, M, and X represent elements of the periodic table and O represents oxygen. R can be one or a combination of Y, Yb, Er, Ho, Eu, Dy, Gd, La, Pr, Sm, Nd; M is one of Ba, Ce or Th; and X can be one of Cu, Bi, K or Ni. In these materials, the atomic ratios of the chemicals R:M:X, i.e., the subscripts a:b:c, are usually in the ratio of 1:2:3. Therefore, these superconductors are generally referred to as the "1-2-3" compounds. However, other formulations having the ratios of 2:4:7 and 1:2:4 have also been discovered.
In the formulations of the form (M.sub.1-x Q.sub.x).sub.a L.sub.b Ca.sub.c Cu.sub.d O.sub.y, the symbol M represents either Pb or Sb, the symbol N represents either Bi or Tl and the symbol L represents Sr or Ba. These superconductors have been found with ratios a:b:c:d of 2:2:n:n+1 where n=(0,1,2,3 . . . ).
All of these various metal oxides become superconductors at low temperatures. However, the most widely used compounds are Y.sub.a Ba.sub.b Cu.sub.c O.sub.y, (Pb.sub.1-x Bi.sub.x).sub.a Sr.sub.b Ca.sub.c Cu.sub.d O.sub.y, and (Pb.sub.1-x Tl.sub.x).sub.a Ba.sub.b Ca.sub.c Cu.sub.d O.sub.y, which have critical temperatures above the boiling temperature of liquid nitrogen at one atmosphere pressure, i.e., (77.3.degree. K.).
It is useful to provide such superconducting oxides in a form suitable for conducting electricity, and particularly, in a form which can be electrically connected to other superconducting elements and non-superconducting (i.e., normal conducting) elements.
As is readily understood, it is vital to be able to join pieces of superconducting material to each other in a way that maintains superconductivity. It is also important to be able to join a piece of superconducting material to a piece of normal conductor in a way that does not adversely effect the superconductivity of the piece of superconducting material. Before the present invention, due to the chemical and mechanical properties of superconductors, it has been difficult to make suitable electrical joints and connections involving superconductors.
For most applications, these materials need to be formed into a wire like structure which in turn is used as a conductive path for electricity. For some applications, wire is wound into coils to generate a magnetic field. A continuous length of thousands of feet or even miles of conductor is needed and it must be superconducting along its entire length. The long lengths must be fabricated by joining two or more shorter lengths end to end. These joints between two lengths must have the same superconducting properties, i.e., critical current, as the wire itself for the joined wire to be useful in the given application.
Furthermore, it is necessary that a technique be available for removing and replacing a section of the wire if it becomes damaged. For example, if a power transmission line is damaged by lightning, the line breaks must be repaired by removing the damaged section of the line and replacing it with a new piece. If the line is made from superconducting wire, the connections between the new section and the undamaged sections of the original line must be superconducting and also have the same current capacity as the original and replacement wires themselves.
It is known how to make an electrically conducting joint between two pieces of a specific type of high temperature superconductor. The superconductor is YBa.sub.2 Cu.sub.3 O.sub.7-x. This type of superconductor is a "1-2-3" superconductor. See generally Y. Tzeng, "Welding of YBa.sub.2 Cu.sub.3 O.sub.7-x Superconductor", Journal of the Electrochemical Society, Volume 136, Number 2, pp 582-583, (February 1989). According to the disclosed technique, two pieces of 1-2-3 superconductor are overlapped and heated to 1100.degree. C., fusing the pieces together. Following the fusion, the material is heated again to form a 1-2-3 phase and is then oxygenated.
The known technique suffers from a number of disadvantages. The high temperature (1100.degree. C.) required to fuse the ceramic pieces together destroys the 1-2-3 phase structure. Above 1015.degree. C., 1-2-3 superconductors decompose upon melting into non-superconducting phases. For example, the YBa.sub.2 Cu.sub.3 O.sub.7-x melts into a 2-1-1 oxide, Y.sub.2 BaCuO.sub.5, and BaCu0.sub.2.Cu0. The reformation to a 1-2-3 phase compound requires a long exposure at a temperature between 900.degree. C. and 950.degree. C., followed by oxygenation at 450.degree. C. to 500.degree. C. Thus, lengthy steps are required. It is also possible that the phase decomposition will have produced a phase situation that will not produce a superconductor at all upon subsequent heat treating.
An additional drawback to this known process is that it cannot be used for a composite of a superconducting ceramic and a noble metal, such as silver, palladium, platinum or gold. By "noble metal," it is meant a metal that will not oxidize at the temperatures applied during certain heat treating conditions discussed below, necessary to create a superconductor. Formation of such a metal/ceramic composite is disclosed in U.S. Pat. No. 4,826,808, to Yurek et al, "Preparation of Superconducting Oxides and Oxide Metal Composites," at examples 2-10. The 4,826,808 patent is hereby incorporated by reference herein.
The presence of the noble metal provides beneficial mechanical properties to the superconducting composite. The composite may be in many forms. The noble metal and ceramic may be in alternating sheets or layers. The noble metal can be present as islands within a matrix of ceramic or vice versa. The noble metal and ceramic can each be present as a continuous phase intimately intermixed within the other, much like a sponge and the voids there through. The known process cannot be used for noble metal/ceramic composites because noble metals may either melt at the high processing temperatures, or agglomerate as solid particles, thereby minimizing or eliminating the desired effects of the noble metal in the composite.
The Yurek patent also discloses generally a method for preparing a superconductor, of either an oxide or a metal/oxide composite. The metallic elements of the oxide in substantially the stoichiometric proportions needed to form the superconducting oxide or composite are combined to form an alloy. The alloy is shaped as desired (or applied to a substrate in some form of coating) and is then oxidized in a heating cycle. The resultant is a superconducting oxide or metal/oxide composite.
Thus, the several objects of the invention include: to provide a process of forming a superconducting joint that under some circumstances does not require long resident times at elevated temperatures; to provide a process of forming a superconducting joint that does not require elevating the composite to temperatures above the melting temperature of the superconducting oxide or of the noble metals such as silver or gold; to provide a process of forming a superconducting joint that uses well-known standard metal joining techniques; to provide a process of forming a superconducting joint that results in a joint that is relatively homogeneous, as compared to the major body of the superconducting material.